Repairing and modifying vintage condensor microphones is the field where I possess the most expertise. All of the vintage Neumann, Telefunken, AKG, Schoeps, SELA, PML, Gefell, Sony etc have been on my workbench.
I also have experience with most of the modern clones as well and can suggest upgrades to all of them, even the expensive boutique models.
I repair other vintage studio gear as well. I prefer tube or older solid state circuits. I don't repair surface mount equipment and digital processors.